690 



ZOOLOGY 



SECT, 



originally posterior, but subsequently becoming shifted round, in 

 the course of the displacement already referred to, to the right- 

 hand side. This covers over a cavity the mantle cavity 



clcn 



FIG. 592. Pterotrachea scutata. nil. alimentary canal ; cten. gills ; eiic, eye ; fl. float ; 

 mo. mouth; pi-ob. proboscis; ,\j>,: reproductive gland; sli. shield covering a portion of the- 

 dorsal surface ; su. sucker. 



situated anteriorly, in which are situated the anal and nephridial 

 apertures and the ctenidia. The edges of the mantle-flap may 

 become united together in such a way as to form a chamber 

 opening on the exterior by a comparatively narrow opening. 

 In many of the Prosobranchia the edges of this aperture are 

 drawn out into a spout-like prolongation open ventrally the 

 siphon which lies in the corresponding prolongation of the peri- 

 stome of the shell, and serves as a channel for the ingress and egress 



oc.tenl 



lent 



pulm, 



FIG. 593. Helix nemoralis . an. anus ; gun. ap. genital aperture ; or. tent, posterior eye-bearing 

 tentacles ; jniliu. opening of pulmonary sac ; tent, anterior tentacles. (After Pelseneer.) 



of water. In some Gastropods, however, there is no definite 

 mantle-cavity, the anus, nephridial apertures, and ctenidia 

 merely lying under cover of a comparatively slightly-developed 

 lateral mantle-flap. Usually there is on the inner surface of the 

 mantle a glandular area the pallial mucus gland. 



Respiratory Organs. There are normally two ctenidia, one on 

 the right side and the other on the left, contained in the mantle- 



